Geobiology最新文献

Methanogenic archaea were likely among the earliest organisms to populate the Earth, perhaps contributing to the Archaean greenhouse effect; they are also widely discussed as analogues to any potential life on Mars. However, fossil evidence of archaea has been difficult to identify in the rock record, perhaps because their preservation potential is intrinsically low or because they are particularly small and difficult to identify. Here, we examined the preservation potential of a methanogen of the genus Methanobacterium, recently isolated from a low-temperature serpentinizing system, an environment somewhat analogous to habitats on the early Earth and Mars. Notably, this organism has a cell wall composed of peptidoglycan-like pseudomurein, which may imply a mineralisation potential similar to that of gram-positive bacteria. Methanobacterium cells were placed in carbonate, phosphate, and silicate solutions for up to 3 months in order to assess the relative tendency of these minerals to encrust and preserve cellular morphology. Cells readily acquired a thick, uniform coating of silica, enhancing their potential for long-term preservation while also increasing overall filament size, an effect that may aid the discovery of fossil archaea while hindering their interpretation. Phosphates precipitated from the medium in all experimental setups and even in parallel experiments set up with low-phosphate medium, suggesting a hitherto unknown biomineralisation capacity of methanogens. Carbonate precipitates did not form in close association with cells.

The microbially mediated replacement of sulfate-bearing evaporites by authigenic carbonate and native sulfur under anoxic conditions is poorly understood. Sulfur-bearing carbonates from the Monte Palco ridge (Sicily) replacing Messinian gypsum were therefore studied to better characterize the involved microorganisms. The lack of (1) sedimentary bedding, (2) lamination, and (3) significant water-column-derived lipid biomarkers in the secondary carbonates implies replacement after gypsum deposition (epigenesis). Allochthonous clasts from the older Calcare di Base and the younger Trubi Formation within these carbonates further evidence epigenetic formation. The sulfur-bearing carbonates are significantly ¹³C-depleted (δ¹³C as low as −51‰), identifying methane as a major carbon source. The ¹⁸O-enrichment of the carbonates (δ¹⁸O as high as 5.4‰) probably reflects precipitation from ¹⁸O-enriched fluids transported along adjacent faults or precipitation in a closed system with very little water. Native sulfur with variable ³⁴S-enrichment (δ³⁴S as high as 18.9‰), a relatively small maximum offset (12.3‰) between the sulfate source (gypsum) and native sulfur, and high δ³⁴S values of carbonate-associated sulfate (as high as 61.1‰) suggest a high conversion to native sulfur in a (semi-)closed system, with insignificant sulfate removal. Anaerobic methanotrophic archaea (ANME) apparently affiliated with the ANME-1 clade mediated secondary mineral formation as evidenced by the biomarker inventory, which contains abundant ¹³C-depleted isoprenoids including sn3-hydroxyarchaeol as the sole hydroxyarchaeol isomer and glycerol dibiphytanyl glycerol tetraethers (GDGTs). A series of various, tentatively identified ¹³C-depleted non-isoprenoidal dialkyl glycerol diethers (DAGEs), 10me-C₁₆ fatty acid, hydroxy C₁₆ fatty acids, and cyclopropyl-C_17:0ω7,8 fatty acid agree with sulfate-reducing bacteria participating in the anaerobic oxidation of methane. Specific conditions during gypsum replacement, unlike those at marine methane seeps, are reflected by the occurrence of ¹³C-depleted lipids such as lycopane, 9me-C₁₇ fatty acid, and novel DAGEs. As a response to a confined environment probably characterized by high sulfate concentrations, sulfidic conditions, and elevated salinity, ANMEs and sulfate-reducing bacteria apparently adapted their membrane compositions to cope with such stressors.

Microbialites are organo-sedimentary structures formed throughout most of the Earth history, over a wide range of geological contexts, and under a multitude of environmental conditions affecting their composition. The carbon and oxygen isotope records of carbonates, which are most often their main constituents, have been used as a widespread tool for paleoenvironmental reconstructions. However, the multiplicity of factors that influence microbialites formation is not always properly distinguished in their isotopic record, in both ancient and modern settings. It is therefore crucial to refine our understanding of the processes controlling microbialites isotopic signal. Here, we analyzed the carbon and oxygen isotope compositions from bulk and micro-drilled carbonates as well as bulk organic carbon isotope compositions in microbialites from four Mexican volcanic crater lakes of increasing alkalinity. The survey of four lakes allows comparing microbialite formation processes and their geochemical record within distinct physico-chemical contexts. The geochemical analyses were performed in parallel to petrographic and mineralogical characterization and interpreted in light of the known microbial community composition for microbialites of the same lakes. Combining these data, we show that the potential for isotopic biosignature preservation primarily depends on physico-chemical conditions. Carbon isotope biosignatures pointing out to an autotrophic influence on carbonate precipitation are preserved in the lowest alkalinity lakes. By contrast, higher alkalinity lakes, where microbialites are more massive, favor carbonate precipitation in isotopic equilibrium with the lake water, with secondary influence of heterotrophic organic carbon degradation. From these results, we suggest that microbialite carbonate C isotope records can be interpreted as the balance between the microbialite net primary productivity and the amount of precipitation that relates to physico-chemical forcing. The signals of microbialite oxygen isotope compositions highlight a lack of understanding in the oxygen isotope records of relatively rare carbonate phases such as hydromagnesite. Nonetheless, we show that these signals are primarily influenced by the basins' hydrology, though biological effects may also play a (minor) role. Overall, both carbon and oxygen isotopic signals may record a mixture of different local/global and biotic/abiotic phenomena, making microbialites intricate archives of their growth environment, which should thus be interpreted with cautions and in the light of their surrounding sediments.

The evolution of oxygenic photosynthesis during the Archean (4–2.5 Ga) required the presence of complementary reducing pathways to maintain the cellular redox balance. While the timing of the evolution of superoxide dismutases (SODs), enzymes that convert superoxide to hydrogen peroxide and O₂, within bacteria and archaea is not resolved, the first SODs appearing in cyanobacteria contained copper and zinc in the reaction center (CuZnSOD). Here, we analyse growth characteristics, SOD gene expression (qRT-PCR) and cellular enzyme activity in the deep branching strain, Pseudanabaena sp. PCC7367, previously demonstrated to release significantly more O₂ under anoxic conditions. The observed significantly higher growth rates (p < 0.001) and protein and glycogen contents (p < 0.05) in anoxically cultured Pseudanabaena PCC7367 compared to control cultures grown under present-day oxygen-rich conditions prompted the following question: Is the growth of Pseudanabaena sp. PCC7367 correlated to atmospheric pO₂ and cellular SOD activity? Expression of sodB (encoding FeSOD) and sodC (encoding CuZnSOD) strongly correlated with medium O₂ levels (p < 0.001). Expression of sodA (encoding MnSOD) correlated significantly to SOD activity during the day (p = 0.019) when medium O₂ concentrations were the highest. The cellular SOD enzyme activity of anoxically grown cultures was significantly higher (p < 0.001) 2 h before the onset of the dark phase compared to O₂-rich growth conditions. The expression of SOD encoding genes was significantly reduced (p < 0.05) under anoxic conditions in stirred cultures, as were medium O₂ levels (p ≤ 0.001), compared to oxic-grown cultures, whereas total cellular SOD activity remained comparable. Our data suggest that increasing pO₂ negatively impacts the viability of early cyanobacteria, possibly by increasing photorespiration. Additionally, the increased expression of superoxide-inactivating genes during the dark phase suggests the increased replacement rates of SODs under modern-day conditions compared to those on early Earth.

Banded iron formations (BIFs) are chemical sedimentary rocks commonly utilized for exploring the chemistry and redox state of the Precambrian ocean. Despite their significance, many aspects regarding the crystallization pathways of iron oxides in BIFs remain loosely constrained. In this study, we combine magnetic properties characterization with high-resolution optical and electron imaging of finely laminated BIFs from the 2.7 Ga Carajás Formation, Brazil, to investigate their nature and potential for preserving ancient environmental conditions. Our findings reveal that magnetite, in the form of large 0.1–0.5 mm crystals, is the main iron oxide, with an overall averaged saturation magnetization (M_s) of 25 Am²/kg (corresponding to ~27 wt% of magnetite) over the studied 230 m of the sequence. Nevertheless, the non-negligible contribution of minerals with higher coercivity suggests variable proportions of hematite along the core. Additionally, we observe non-uniform behavior in magnetite grains, with distinct populations identified through low-temperature measurements of the Verwey transition. Petrographic observations indicate that the original sediment was an Fe–Si mud consisting of a ferrihydrite–silica mixture formed in the water column. This assemblage was rapidly transformed into nano-scale hematite embedded in silica as indicated by a honeycomb structure composed of Si-spherules distributed in a microscale hematite matrix. Textural relationships show that the nucleation of magnetite started during or soon after the formation of hematite, as indicated by the preservation of the Si-spherules within magnetite cores. Further magnetite overgrowth stages are characterized by inclusion-free rims, associated with continuous Si supply during the evolving diagenetic or early metamorphic stages. These findings, combined with existing literature, suggest that ferrihydrite precipitated alongside Si and organic material, later crystallizing as hematite on the seafloor. Anaerobic respiration by Fe(III)-reducing microorganisms likely contributed to early magnetite formation in a fluid-saturated, unconsolidated sediment. Subsequent low-grade metamorphism and Si mobilization led to palisade quartz precipitation and a second stage of magnetite growth likely formed at the expense of matrix hematite through thermochemical Fe(III) reduction. Low-temperature magnetic analyses revealed that the two generations of magnetite core and rim are associated with specific stoichiometry.